An example of the above-described conventional electromagnetic clutch is disclosed in Patent Document 1, in which a driving rotary element (a first element in Patent Document 1) and a driven rotary element (a second element in Patent Document 1) are provided adjacent to each other along a rotational axis to be rotatable coaxially with the rotational axis. A wrap spring is arranged in outer circumferences of the driving rotary element and the driven rotary element. A control collar is provided around the wrap spring. One end of the wrap spring is supported to the driving rotary element and the other end of the wrap spring is supported to the control collar.
According to Patent Document 1, a clutch-disengaging state is achieved when a magnetic force is not exerted on an armature provided in the control collar, in which the wrap spring is separated from the outer circumferences of the driving rotary element and the driven rotary element. When the magnetic force is exerted on the armature provided in the control collar to reduce a winding diameter of the wrap spring and bring the wrap spring into pressure contact with the outer circumferences of the driving rotary element and the driven rotary element, thereby to transmit a rotational driving force of the driving rotary element to the driven rotary element.
Meanwhile, Patent Document 2 discloses an arrangement including a driving rotary element (an input pulley in Patent Document 2), a driven rotary element (an output hub in Patent Document 2), and a wrap spring provided in outer circumferences of winding surfaces of the driving rotary element and the driven rotary element. One end of the wrap spring is supported to an armature, and the other end of the wrap spring is supported to the driven rotary element. An electromagnetic solenoid is mounted within the driving rotary element for exerting a drawing force on the armature.
According to Patent Document 2, a clutch-disengaging state is achieved when a magnetic force from the electromagnetic solenoid is not exerted on the armature, in which the wrap spring is separated from the outer circumferences of the input pulley and the output hub. When the magnetic force is exerted from the electromagnetic solenoid to the armature for drawing the armature, the wrap spring comes into tight contact with the outer circumferences of the input pulley and the output hub, thereby to transmit a driving force from the driving rotary element to the driven rotary element.